Induction heating of elongated bars

ABSTRACT

There is disclosed herein a method and means for heating a semicontinuous line of elongated bars passing through a relatively shorter induction coil whereby the ends of the bars will be heated to substantially the same temperature as the midportions thereof. A signal from the generator current is directed to a regulating device which maintains the generator current at predetermined values during periods when the lead end portion of a bar is entering or the trailing-end portion of a bar is leaving the coil and during passage of the bar through the coil, the heating of the bar being controlled by voltage regulation when the midportion of a bar is passing through the coil.

United States Patent Appl. No. Filed Patented Assignee INDUCTION HEATING 0F ELONGATED BARS 9 Claims, 6 Drawing Figs.

US. Cl 219/1077, 219/1041, 219/1069 1nt.Cl 1105b 5/00, H05b 1/02 Field of Search 219/10.77, 10.41

Primary ExaminerJ. V. Truhe Assistant Examiner-L. H. Bender Attorney-J. H. Slough ABSTRACT: There is disclosed herein a method and means for heating a semicontinuous line of elongated bars passing through a relatively shorter induction coil whereby the ends of the bars will be heated to substantially the same temperature as the midportions thereof. A signal from the generator current is directed to a regulating device which maintains the generator current at predetermined values during periods when the lead end portion of a bar is entering or the trailingend portion of a bar is leaving the coil and during passage of the bar through the coil, the heating of the bar being controlled by voltage regulation when the midportion of a bar is passing through the coil.

oooeoP O JOOO/OOOO INDUCTION HEATING F ELONGATED BARS This invention relates to the induction heating of elongated workpieces such as bars which are moved endwise through an induction heater coil of shorter axial dimension than the bars and wherein the line of bars is semicontinuous; that is, the bars are not positioned end to end, but each bar is spaced from both the preceding and succeeding bars.

The conventional means for controlling the voltage to the induction coil is by means of a voltage regulator. In the case of semicontinuous induction heating, the coil is energized when partly loaded as the bar enters and also under similar conditions as the bar leaves the coil. This makes it difficult to keep the ends of the bar at the same temperature as the middle portions, the tendency being to overheat the ends which only partly fill the coil as the bar enters or leaves the coil. Also, with certain workpieces, not necessarily bars of uniform cross section throughout, there may be a tendency to overheat the ends. The basic problem which the present invention solves is the lack of uniformity of temperature at the ends of the bars or workpieces heated in this manner due to the changing affect on the load when the coil is only partially filled. These problems of nonuniforrnity occur both at the leading and trailing ends of the bars.

In the heating of each bar, there are three distinct periods presenting distinctly different heating conditions, the problem of each period being different from the other two. The first period is when the bar is first entering and progressively filling the coil at which time the bar is at ambient or some other, preheated temperature. The second period is from the time that the coil is filled with the bar until the trailing end of the bar begins to enter the coil. The final period of heating is during the time when the trailing end portion of the bar is leaving the coil. At this point a substantial amount of the bar has been heated and the heating of the trailing end portion thereof as it progressively leaves the coil will nonnally call for a different control than that covering the leading end thereof.

The present invention as herein disclosed and illustrated provides means for sensing or detecting the ends of the bar and switching to appropriate current regulation for a timed period during the first and last periods when end portions of the bar are either entering or leaving the coil. A signal directly from the generator current is directed to a regulating device which acts on the field of the generator and maintains the generator current at a predetermined value or values during passage of the bar through the coil. Under these circumstances, the voltage will rise and fall as the coil is being either filled or emptied of the bar, in proportion to the amount of bar in the coil, thereby controlling the amount of voltage applied to the bar.

In view of the foregoing, it is the primary object of this invention to provide an improved means and method for unifonnly heating an elongated bar or workpiece in a coil of relatively smaller axial dimension than said bar.

Another object of the invention is to provide means for effectively heating the ends of the bar as they enter or leave the coil to provide power to the coil in proportion to the amount of bar disposed in the coil at a given time.

Still another object of the invention is to provide means for automatically switching between voltage regulation and current regulation as the bar assumes different positions with respect to the coil.

Yet another object of the invention is to provide an elongated bar-heating means wherein the ends of the bar are heated under conditions of current regulation whereas the midportions of the bars are heated by voltage regulation.

A still further object of the invention is to provide an automatic control means for automatically heating successive elongated bars disposed in a semicontinuous line; that is, with the bars being spaced from each other whereby there is a period between the heating of each successive bar when the coil is empty.

Other objects and advantages of this invention will be readily apparent from the following detailed description of the invention and the accompanying drawings, in which said drawings:

FIG. I is a simplified drawing of an elongated bar at the moment of entering into an induction-heating coil, the bar being shown partially disposed within the coil in broken lines;

FIG. 2 is similar to FIG. I and shows the bar in a second position wherein the coil is entirely filled;

FIG. 3 shows a third position of the bar wherein the trailing end of the bar has just reached the entrance to the coil;

FIG. 4 shows a fourth position of the bar wherein the trailing end of the bar has just left the exit end of the coil, the trailing end being shown partially disposed within the coil in broken lines;

FIG. 5 is a diagram indicating the manner in which the voltage rises, is maintained, and falls as a bar passes through the coil; and

FIG. 6 is a simplified electrical diagram showing an automatic control system for heating successive bars in a semicontinuous production line.

Referring now to the drawings in all of which like parts are designated by like reference numerals, the simplified drawings of FIGS. 1-4 show an induction coil 10 adapted to receive and heat an elongated bar 11. The bar 11 represents one workpiece in a semicontinuous line of spaced workpieces adapted to move axially or in the direction of the arrow 12 along a powered conveyor 13 through the coil 10. It will be noted that each bar 11 is substantially longer than the coil 10 in the axial direction whereby it is impossible for the said coil to heat the entire bar at one time. Assuming that the bar II is moving in the direction of the arrow 12 at a constant rate, it will be readily understood that there will be a period between the position shown in FIG. 1 and that shown in FIG. 2 when the coil 10 is only partially filled, as shown in dotted lines in FIG. I, with the leading end portion of the bar as the bar moves into said coil. It will be further readily seen that between the position of FIG. 2 and that shown in FIGS, the coil 10 will be completely filled with the bar 1 1 whereas between the positions shown in FIG. 3 and FIG. 4, the trailing end portion will be progressively leaving the coil thereby progressively reducing the amount of bar being heated, as shown by dotted lines in FIG. 4. Thus there are three separate conditions under which the coil must heat the bar: the entering condition, as shown by the dotted lines in FIG. 1, between the full-line positions shown in FIGS. I and 2, the full condition between the positions shown in FIGS. 2 and 3, and the emptying condition when the bar is, as indicated by the dotted lines in FIG. 4, between the positions shown in FIGS. 3 and 4.

FIGS. 1-4 also show in diagrammatic form a double-pole switch LS which will be understood to be the same switch LS as that shown in the electrical diagram of FIG. 6. The switch LS is disposed just outside the entrance end of the coil 10 and is tripped at the first position when the bar I] is about to enter the coil 10, is held in the tripped condition through the second position shown in FIG. 2, and automatically reopens or returns to its normal position when the bar reaches substantially the third position shown in FIG. 3.

FIG. 6 is a simplified electrical diagram indicating the manner in which current and voltage are controlled to control the heating pattern applied to the bar 11 whereby the same will be heated unifonnly throughout. All of the relay contacts of the control circuitry shown at the left-hand side of the electrical diagram are shown in their normal condition to which they automatically return when their respective relays are deenergizcd, and it will be understood that energization of the corresponding relays causes the contacts they control to assume the opposite position. It will be further understood that the timer contacts may be actuated a timed period after energization of the timer relays to provide optimum positioning of the bar within the coil before current regulation is begun. Because the present electrical diagram has been simplified. each relay referred to may, in fact, represent plural. interrelated relays or other known electrical control and switching means. Also, many safety interlocks and controls not specifically related to the switching in and out of control means have been eliminated.

In FIG. 6, L1, L2, and L3 represent the main leads to a three-phase supply, the same being connected to a motor M connected to a generator G. A control transformer 15 is connected across the leads L1 and L3 and provides a control circuit to lines 16 and 17. Current from the generator G is directed through lines 18 and 19 to the primary of a highfrequency transformer 20, the secondary of which is directed through lines 21 and 22 to the inductor I0. Capacitors 23 in lines 24 are connected in parallel with the inductor l and may be controlled by switches 25. A voltage regulator and voltage and current adjustment control device 26 is adapted to receive a signal from the generator current by means of a coil 27 encircling the line 18 leading from said generator. The control device 26 is also connected to the secondary of a voltageregulating transformer 28, connected across the lines 18 and 19.

In operation of the control means for controlling the heating of bar 11, a main switch 30 in the leads L1 and L3 is closed to direct current to the main control lines 16 and 17. A manual start switch 31 is then closed to energize a starter relay S across line 32. The relay S represents motor energizing and sustaining means whereby the motor M and generator G are started up to supply current to the inductor 10.. The relay S closes holding contacts 8-1 in bypass line 33 around the start switch 31 to maintain the current to said relay. A normally closed stop switch 34 is also disposed in the line 32 which upon opening will deenergize the relay S which will immediately be isolated by opening of the contacts 8-1 to stop the motor and generator.

The inductor is energized for heating the bars 11 by a suitable heat relay I-I disposed in a line 35 connected across the control lines 16 and 17. The line 35 is provided with a normally open start switch 36 and a normally closed stop switch 37, hold-in contacts I-I-l being provided in a bypass line 38 around the start switch 36. A pair of normally open contacts TEC-2 are also interposed in the line 35 and would be closed at the time the start switch 36 is actuated as will hereinafter be fully explained.

As shown in FIGS. 1-4, each moving bar 1 1 in the semicontinuous line of bars will, in turn, strike the double-pole switch LS. The switch LS is connected to the control line 16 at the line 39 and has two sets of contacts: normally open contacts LS-l in a line 40 controlling a leading-end timer relay LET, and normally closed contacts LS-2 in a line 41 controlling a trailing-end timer relay TET. Thus, prior to any bar 11 entering the inductor coil 10, the contacts LS-l will be open and the contacts LS-2 will be closed. Therefore, whenever current is supplied to the control lines 16 and 17 by closing the main switch 30, the trailing end timer relay TET will normally be energized. Energization of relay TET closes normally open contacts TET-l in a line 42 disposed across the control lines I6 and 17 and having a trailing-end current regulation relay TEC interposed therein. Energization of the relay TEC, in turn, closes normally open contacts TEC-2 in line 35 previously referred to whereby the start switch 36 can be closed to energize the relay H which will close the hold-in contacts H-l as hereabove described. Energization of the trailing end current regulation relay TEC also opens normally closed contacts TEC1 in a line 43 connected across the control lines and having a voltage-regulating relay VR interposed therein. The relay VR is adapted to control the voltage at a constant level during the time when the bar 11 fill the coil 10 as when said bar is moved from the position shown in FIG. 2 to substantially the position shown in FIG. 3. The leading end timer relay LET in line 40 controls a pair of normally open contacts LET-4 disposed in a line 44 connected across the control lines 16 and 17 and having a leading end current regulation relay LEC interposed therein. Because the contacts LS-l of the doublepole switch LS are initially open, the leading-end or first timer relay LET will be initially deenergized whereby the contacts LET-l will be open and the leading-end current relay LEC will be isolated. The voltage control relay VR is also isolated by the now open contacts TEC-l whereby the empty coil is controlled by the trailing-end current relay TEC only.

When a moving bar 11 strikes the switch LS, normally open contacts LS-l are closed to energize the relay LET of the first timer and the normally closed contacts LS-2 are opened to isolate and deenergize the trailing-end or second timer relay TET. The relay LET closes its normally open contacts LET-1 in line 44 thereby energizing the leading end current relay LEC. The contacts TET-l in line 42 now open to isolate the trailing-end current relay TEC whereby the contacts TEC-l in line 43 return to their closed position. However, the voltageregulating relay VR is maintained in the isolated condition because the line 43 also has disposed therein a pair of normally closed contacts LEC-l which open immediately upon energization of the leading end current relay LEC. At this point, a signal from the generator current is directed to the control device 26 which now acts on the field of the generator to maintain the generator current at a predetermined value for optimum heating of the leading-end portion of the'bar as it enters the inductor coil 10.

When the first timer times out, the leading-end timer relay LET is automatically deenergized thereby isolating the leading-end current relay LEC by allowing the normally open contacts LET-l to open. At the same time, the normally closed contacts LEC-1 are allowed to close thereby completing a circuit across the line 43 to energize the voltage-regulating relay VR. At this time, the control device 26 takes a signal from the secondary of the voltage-regulating transformer 28, compares it to a constant reference voltage, and then acts on the field of the generator to maintain the voltage constant at a predetermined level. The control device 26 also picks up a signal from the generator current through the coil 27 and holds the current at a constant predetermined value whereby there is both voltage and current regulation during the time when the inductor coil 10 is being filled with a bar 11.

When the trailing end of each bar 11 approaches the en- V trance end of the coil 10, said bar rides ofi" the double-pole switch LS whereby contacts LS-l return to the open position and contacts LS-2 return to the closed position as illustrated in FIG. 6. The second timer relay TET is then energized and after a short time delay which allows the trailing end of the bar to reach the entrance to the coil 10, the contacts TET-I in line 42 close to energize the trailing-end current regulation relay TEC. It will be readily seen that at this time, relays LET, LEC, and VR are isolated, the last-mentioned relay being isolated by the opening of normally closed relay contacts TEC-1. The control device 26 again picks up a signal directly from the generator current through the coil 27 and acts on the field of the generator to maintain the generator current at a predetermined value as the trailing-end portion of the bar is withdrawn from the inductor coil 10. The next succeeding bar 11 will again actuate the double-pole switch LS and repeat the heating cycle for the next bar.

During heating of the leading and trailing ends of the bars, the generator current may be held at a predetermined constant value or it may be caused to vary or swing through an arc of predetermined values whereby the optimum heating conditions are obtained for the particular bar or workpiece to be heated. Such optimum conditions of constant or predetermined varied values would be obtained by experimentation with any given bar or workpiece. As hereinabove described, the remainder of the time when the coil is full as the bar passes therethrough, both current and voltage are maintained at a constant value.

As a substitute for the control device 26 and its circuitry, a programmed numerical computer can be used to control the temperature regulation and resultant temperature of the bars, and it will be readily appreciated that this will eliminate need for timing mechanism, the current regulation being dependent upon distance rather than timing.

FIG. 5 represents diagrammatically the voltage level throughout the heating of a single bar 11. The vertical axis E represents the voltage level and the horizontal axis represents the time elapsed and the distance covered by the traveling bar. The numerals I, 2, 3, and 4 of the horizontal axis correspond to the positions I, 2, 3, and 4 shown in full lines in the FIGS. 1.

2, 3, and 4, respectively. It will be seen that during the time interval between positions 1 and 2 during which the generator current is maintained at predetermined values, the voltage swings upwardly with a greater amount of bar entering the inductor coil. Between positions 2 and 3 when voltage regulation is being used, the voltage is maintained constant at a desired level and the generator current is maintained at a constant value. Between positions 3 and 4 a return to control of the generator current is made at which point the voltage drops off or swings downwardly as the bar leaves the inductor coil. it will be noted that the voltage patterns in the areas designated by positions 1 and 2 and positions 3 and 4 vary and that, therefore, the leading end portion and the trailing-end portion are preferably separately controlled to attain the desired uniform heating pattern. It will be further understood that the voltage patterns in the areas designated as positions 1 and 2 and positions 3 and 4 can be varied from that shown to provide optimum heating patterns for workpieces of different shapes.

Thus the present invention provides effective means for eliminating the nonuniformity in the heating of bars of the type and in the manner herein disclosed. It enables the user to take advantage of the built-in protection of conventional voltage regulation throughout the midportion of the bar while at the same time eliminating the danger of overheating or underheating the ends thereof.

It will be understood that many changes in the details of the invention as herein described and illustrated may be made without, however, departing from the spirit thereof or the scope of the appended claims.

We claim:

1. Induction-heating means for uniformly inductively heating an elongated workpiece, said induction-heating means comprising an inductor coil of substantially shorter axial dimension than the workpiece to be heated; a source of electrical power for said coil including a generator; means for moving the elongated workpiece endwise through said coil generally along the axis of said coil; means for causing the voltage in the coil to rise proportionally as the leading-end portion of each workpiece enters said coil; means for maintaining the voltage at a predetermined level and the generator current at a constant value substantially during the time the coil is full; and means for causing the voltage to drop off proportionally as the trailing end portion of the workpiece approaches the exit end of the coil whereby the generator current is maintained at predetermined values when any portion of the workpiece is contained within said inductor coil and the workpiece is substantially uniformly heated through the length thereof.

2. Induction-heating means for uniformly inductively heating an elongated bar, said induction-heating means comprising an inductor coil of substantially shorter axial dimension than the bar to be heated; a source of electrical power for said coil including a generator; means for moving the elongated bar endwise through said coil generally along the axis of said coil; means for maintaining the generator current constant at a predetermined value when the leading-end portion of the bar is entering and filling the coil; means for maintaining the voltage at a predetennined level and the generator current at a constant value substantially during the time the coil is full; and means for maintaining the generator current constant at a predetermined value when the trailing end portion of the bar is being withdrawn from the coil, whereby the bar is substantially uniformly heated throughout the length thereof.

3. Induction-heating means for uniformly heating an elongated bar comprising an inductor coil of substantially shorter axial dimension than the bar; a source of electrical power for said coil including a generator; means for moving the elongated bar endwise through said coil generally along the axis of said coil; control means for regulating the current and voltage to said coil; trip means located adjacent to the entrance end of said coil and tripped by the bar as it enters and progressively fills said coil; said control means responsive to said trip means to maintain the generator current constant at a predetermined value substantially during the time the bar is progressively filling said coil; said control means responsive to said trip means to maintain a predetermined voltage to said coil during the time said coil is full and until the trailing-end portion of the bar progressively leaves the coil; said control means responsive to the bar moving away from said trip means to maintain the generator current at a constant predetennined value substantially during the time when the bar is being progressively withdrawn from said coil, whereby the bar is substantially uniformly heated throughout the length thereof.

4. Induction-heating means for uniformly heating an elongated bar as set forth in claim 3: said induction-heating means adapted to automatically successively heat a semicontinuous line of spaced, elongated bars, said trip means comprising a switch actuated by the leading end of each bar and released as the trailing end of each bar rides off of said switch as substantially the time said trailing end enters said coil; said control means including a first timer initiating control of the generator current during the time the bar is progressively filling said coil and a second timer initiating control of the generator current during the time the bar is being progressively withdrawn from said coil.

5. Induction-heating means for uniformly heating an elongated bar as set forth in claim 3: said source of electrical power for said coil including a high-frequency transformer; said generator connected to the primary of said transformer and said inductor coil connected to the secondary of said transformer; capacitors connected in parallel with said inductor coil; said means for maintaining the generator current constant at a predetermined value being responsive to sensing means located between said generator and said transfonner.

6. The method of heating elongated workpieces in an inducted coil of substantially shorter length than the workpiece, the method comprising energizing the coil by means including a generator, passing the workpieces endwise through the coil in a semicontinuous line with the workpieces spaced apart a sufficient distance whereby the coil will be at least momentarily empty between workpieces, causing the voltage in the coil to rise proportionally as the leading end portion of each workpiece enters the coil; maintaining the voltage at a predetermined level and the generator current at a constant value substantially during the time the coil is full; and causing the voltage to drop off proportionally as the trailing-end portion of each workpiece approaches the exit end of the coil whereby the generator current is maintained at predetermined values during passage of the workpieces through the coil and each workpiece is substantially uniformly heated throughout the length thereof.

7. The method of heating elongated bars in an inductor coil of substantially shorter length than the bars, the method comprising energizing the coil by means including a generator, passing the bars endwise through the coil in a semicontinuous line with the bars spaced apart a sufficient distance whereby the coil will be at least momentarily empty between bars, maintaining the generator current constant at a predetermined value sufficient to heat the leading-end portion of each bar to the desired temperature whereby the voltage will rise proportionally as the bar fills the coil, maintaining the voltage at a predetermined level sufiicient to heat the midportion of the bar to the desired temperature while the coil remains full, and maintaining the generator current constant at a predetermined value sufficient to heat the trailing-end portion of the bar to the desired temperature whereby the voltage will drop off proportionally as the bar is withdrawn from the coil and whereby the bar is substantially uniformly heated throughout the length thereof.

8. Induction-heating means for uniformly inductively heating an elongated workpiece, said induction-heating means comprising an inductor coil of substantially shorter axial dimension than the workpiece to be heated; a source of electrical power for said coil including a generator; means for moving the elongated workpiece endwise through said coil parallel with the axis of said coil, there being a first period when said workpiece is filling said coil, a second period during which said coil is full, and a third period when the trailing end portion of the workpiece is leaving said coil; means for maintaining the voltage in said coil at a predetermined level and for maintaining the generator current at a constant value during said second period when said coil is full; and means for maintaining said generator current at predetermined values during said first and third periods whereby the voltage rises and falls proportionally as the workpiece enters and leaves the coil to substantially uniformly heat the workpiece throughout the length thereof.

9. The method of uniformly heating an elongated workpiece in an inductor coil of substantially shorter length than the workpiece, the method comprising providing a source of electrical power for energizing the coil including a generator; moving the workpiece endwise through the coil at a uniform speed whereby there is a first period when said workpiece is filling said coil, a second period during which said coil is full, and a third period when the trailing-end portion of the workpiece is leaving said coil; maintaining the voltage in said coil at a predetermined level and maintaining the generator current at a constant value during said second period when the coil is full; and maintaining said generator current at predetermined values during said first and third periods whereby the voltage rises and falls proportionally as the workpiece enters and leaves the coil to substantially uniformly heat the workpiece throughout the length thereof. 

1. Induction-heating means for uniformly inductively heating an elongated workpiece, said induction-heating means comprising an inductor coil of substantially shorter axial dimension than the workpiece to be heated; a source of electrical power for said coil including a generator; means for moving the elongated workpiece endwise through said coil generally along the axis of said coil; means for causing the voltage in the coil to rise proportionally as the leading-end portion of each workpiece enters said coil; means for maintaining the voltage at a predetermined level and the generator current at a constant value substantially during the time the coil is full; and means for causing the voltage to drop off proportionally as the trailing end portion of the workpiece approaches the exit end of the coil whereby the generator current is maintained at predetermined values when any portion of the workpiece is contained within said inductor coil and the workpiece is substantially uniformly heated through the length thereof.
 2. Induction-heating means for uniformly inductively heating an elongated bar, said induction-heating means comprising an inductor coil of substantially shorter axial dimension than the bar to be heated; a source of electrical power for said coil including a generator; means for moving the elongated bar endwise through said coil generally along the axis of said coil; means for maintaining the generator current constant at a predetermined value when the leading-end portion of the bar is entering and filling the coil; means for maintaining the voltage at a predetermined level and the generator current at a constant value substantially during the time the coil is full; and means for maintaining the generator current constant at a predetermined value when the trailing end portion of the bar is being withdrawn from the coil, whereby the bar is substantially uniformly heated throughout the length thereof.
 3. Induction-heating means for uniformly heating an elongated bar comprising an inductor coil of substantially shorter axial dimension than the bar; a source of electrical power for said coil including a generator; means for moving the elongated bar endwise through said coil generally along the axis of said coil; control means for regulating the current and voltage to said coil; trip means located adjacent to the entrance end of said coil and tripped by the bar as it enters and progressively fills said coil; said control means responsive to said trip means to maintain the generator current constant at a predetermined value substantially during the time the bar is progressively filling said coil; said control means responsive to said trip means to maintain a predetermined voltage to said coil during the time said coil is full and until the trailing-end portion of the bar progressively leaves the coil; said control means responsive to the bar moving away from said trip means to maintain the generator current at a constant predetermined value substantially during the time when the bar is being progressively withdrawn from said coil, whereby the bar is substantially uniformly heated throughout the length thereof.
 4. Induction-heating means for uniformly heating an elongated bar as set forth in claim 3: said induction-heating means adapted to automatically successively heat a semicontinuous line of spaced, elongated bars, said trip means comprising a switch actuated by the leading end of each bar and released as the trailing end of each bar rides off of said swItch as substantially the time said trailing end enters said coil; said control means including a first timer initiating control of the generator current during the time the bar is progressively filling said coil and a second timer initiating control of the generator current during the time the bar is being progressively withdrawn from said coil.
 5. Induction-heating means for uniformly heating an elongated bar as set forth in claim 3: said source of electrical power for said coil including a high-frequency transformer; said generator connected to the primary of said transformer and said inductor coil connected to the secondary of said transformer; capacitors connected in parallel with said inductor coil; said means for maintaining the generator current constant at a predetermined value being responsive to sensing means located between said generator and said transformer.
 6. The method of heating elongated workpieces in an inducted coil of substantially shorter length than the workpiece, the method comprising energizing the coil by means including a generator, passing the workpieces endwise through the coil in a semicontinuous line with the workpieces spaced apart a sufficient distance whereby the coil will be at least momentarily empty between workpieces, causing the voltage in the coil to rise proportionally as the leading end portion of each workpiece enters the coil; maintaining the voltage at a predetermined level and the generator current at a constant value substantially during the time the coil is full; and causing the voltage to drop off proportionally as the trailing-end portion of each workpiece approaches the exit end of the coil whereby the generator current is maintained at predetermined values during passage of the workpieces through the coil and each workpiece is substantially uniformly heated throughout the length thereof.
 7. The method of heating elongated bars in an inductor coil of substantially shorter length than the bars, the method comprising energizing the coil by means including a generator, passing the bars endwise through the coil in a semicontinuous line with the bars spaced apart a sufficient distance whereby the coil will be at least momentarily empty between bars, maintaining the generator current constant at a predetermined value sufficient to heat the leading-end portion of each bar to the desired temperature whereby the voltage will rise proportionally as the bar fills the coil, maintaining the voltage at a predetermined level sufficient to heat the midportion of the bar to the desired temperature while the coil remains full, and maintaining the generator current constant at a predetermined value sufficient to heat the trailing-end portion of the bar to the desired temperature whereby the voltage will drop off proportionally as the bar is withdrawn from the coil and whereby the bar is substantially uniformly heated throughout the length thereof.
 8. Induction-heating means for uniformly inductively heating an elongated workpiece, said induction-heating means comprising an inductor coil of substantially shorter axial dimension than the workpiece to be heated; a source of electrical power for said coil including a generator; means for moving the elongated workpiece endwise through said coil parallel with the axis of said coil, there being a first period when said workpiece is filling said coil, a second period during which said coil is full, and a third period when the trailing end portion of the workpiece is leaving said coil; means for maintaining the voltage in said coil at a predetermined level and for maintaining the generator current at a constant value during said second period when said coil is full; and means for maintaining said generator current at predetermined values during said first and third periods whereby the voltage rises and falls proportionally as the workpiece enters and leaves the coil to substantially uniformly heat the workpiece throughout the length thereof.
 9. The method of uniformly heating an elongatEd workpiece in an inductor coil of substantially shorter length than the workpiece, the method comprising providing a source of electrical power for energizing the coil including a generator; moving the workpiece endwise through the coil at a uniform speed whereby there is a first period when said workpiece is filling said coil, a second period during which said coil is full, and a third period when the trailing-end portion of the workpiece is leaving said coil; maintaining the voltage in said coil at a predetermined level and maintaining the generator current at a constant value during said second period when the coil is full; and maintaining said generator current at predetermined values during said first and third periods whereby the voltage rises and falls proportionally as the workpiece enters and leaves the coil to substantially uniformly heat the workpiece throughout the length thereof. 